Author: krasimir Date: Wed Nov 8 05:28:53 2017 New Revision: 317689 URL: http://llvm.org/viewvc/llvm-project?rev=317689&view=rev Log: [clang-tidy] Add a note about modernize-replace-random-shuffle
Summary: This adds a note warning the users that the way the suggested fix seeds the random number generator is poor. Reviewers: hokein Reviewed By: hokein Subscribers: cfe-commits, xazax.hun Differential Revision: https://reviews.llvm.org/D39787 Modified: clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst Modified: clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst URL: http://llvm.org/viewvc/llvm-project/clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst?rev=317689&r1=317688&r2=317689&view=diff ============================================================================== --- clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst (original) +++ clang-tools-extra/trunk/docs/clang-tidy/checks/modernize-replace-random-shuffle.rst Wed Nov 8 05:28:53 2017 @@ -26,3 +26,16 @@ Both of these examples will be replaced The second example will also receive a warning that ``randomFunc`` is no longer supported in the same way as before so if the user wants the same functionality, the user will need to change the implementation of the ``randomFunc``. One thing to be aware of here is that ``std::random_device`` is quite expensive to initialize. So if you are using the code in a performance critical place, you probably want to initialize it elsewhere. +Another thing is that the seeding quality of the suggested fix is quite poor: ``std::mt19937`` has an internal state of 624 32-bit integers, but is only seeded with a single integer. So if you require +higher quality randomness, you should consider seeding better, for example: + +.. code-block:: c++ + + std::shuffle(v.begin(), v.end(), []() { + std::mt19937::result_type seeds[std::mt19937::state_size]; + std::random_device device; + std::uniform_int_distribution<typename std::mt19937::result_type> dist; + std::generate(std::begin(seeds), std::end(seeds), [&] { return dist(device); }); + std::seed_seq seq(std::begin(seeds), std::end(seeds)); + return std::mt19937(seq); + }()); _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
